Hearing test system



23,1956 R. E. ALLISON HEARING TEST SYSTEM 2 Sheets-Sheet 2 FIG.2

n2 2: I09 105 us m B I N v E N T o R 234 RALPH E ALLISON ATTORNEYS United States PatentC HEARING TEST SYSTEM Ralph E. Allison, Whittier, Califi, assignor to Patent Management, Incorporated, Minneapolis, Minn., a corporation of Minnesota Application December 6, 1950, Serial No. 199,518

15 Claims. (Cl. 179-1) The herein disclosed invention relates to hearing test systems and methods of testing hearing acuity and has for an object to provide a system and method by means of which groups of persons may be simultaneously tested without the use of individual ear phones or similar receivers.

Another object of the invention resides in providing a system using one or more loud speakers and in which the intensity of tone produced by the speakers is substantially the same at all localities where the persons to be tested are stationed.

Another object of the invention resides in conducting the test in a room having walls adapted to reflect sounds and in which the loud speakers are arranged in the room to direct the sound toward one of the walls of the room.

An object of the invention resides in utilizing in large rooms a plurality of loud speakers and in arranging some of said speakers adjacent one of the walls of the room and emanating sound waves directed toward the opposite wall and in arranging other of said speakers intermediate said walls and emanating sound waves directed toward the same wall.

A still further object of the invention resides in using an audiometer for actuating the loud speakers and in providing the audiometer with a generator for producing a tone having a warble whereby the frequency of the reflected sound waves continuously vary within limits so that when the wave from a reflected train of waves reaches the waves directly emanating from the loud speakers the sounds produced by the combination of the direct and reflected Waves is not appreciably varied in intensity and thus standing waves are prevented with the result that the intensity of the sound is approximately the same throughout the room in which the test is being conducted.

Another object of the invention resides in constructing the generator with an oscillator producing alternating currents of various frequencies and with a frequency modulating device for modulating the currents to produce in the loud speakers tones having a warble.

A still further object of the invention resides in providing a current interrupting device by means of which the currents are formed into current pulses producing tone pulses arranged according to predetermined sound patterns.

An object of the invention resides in the method of testing hearing acuity which consists in utilizing sounds adapted to be simultaneously heard at all of the localities where the persons tested are stationed and in applying ear closures to the ears of the persons being tested alternating from one ear of each person to the other.

Other objects of the invention reside in the novel combination and arrangement of parts and in the details of construction hereinafter illustrated and/or described.

In the drawings:

Fig. l is a plan view of a room illustrating the arrangement of the equipment used with the system.

Fig. 2 is a wiring diagram of the audiometer used with the system.

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Fig. 3 is a wiring diagram of a portion of an audiometer showing a modification of the invention.

In the drawings has been illustrated a room in which tests for hearing acuity may be conducted. This room has a front wall, a rear wall and side walls and these walls, particularly the rear wall are constructed to reflect sound waves. In this room have been shown seats for the patients and four loud speakers for producing the desired tones. These speakers are arranged, two near the front wall and two intermediate the front wall and rear wall and all four emanating sound waves directed toward the rear wall. The audiometer used with the system utilizes a triode oscillator of the variable frequency type including a number of [condensers and a tapped coil adapted to be connected to produce the desired frequencies by means of a selector switch. This oscillator is connected to a bank of equalizer resistors which are adapted to be selectively connected in the circuit by means of a multiple contact switch simultaneously operable with the selector switch. The output of the oscillator is fed to an amplifier which in turn is connected to an attenuator. This attenuator controls the output of the amplifier which energizes the loud speakers arranged in the room in which the test is being conducted. A frequency modulating device for modulating the alternating current produced by the oscillator is employed. This device consists of a number of condensers of differing capacities adapted to be connected to the resonating circuit of the oscillator by means of a rotary multiple contact switch rotated from an electric motor through a speed reducing transmission. An interrupter is also employed which is similarly driven and which converts the modulated alternating current into current pulses, producing in the loud speakers warbled tone pulses conforming to any of a number of desired sound patterns.

The herein disclosed invention overcomes certain objections to a similar system for testing hearing acuity and disclosed in the application for patent of Scott N. Reger, Serial No. 697,822, filed September 18, 1948, for Audiometers. The Reger invention utilizes a number of ear phones, one for each person being tested. This requires a great amount of wiring with numerous flexible cords and a great many receivers. In addition the time required to set up such apparatus, in a schoolroom for example, is great and the general commotion and confusion occurring, particularly when school children are to be tested, makes the conducting of the tests an arduous task. The use of loud speakers in place of ear phones occurred to applicant but several problems arose. In the first place it became impossible to procure the same sound intensity at all locations in a room Where the persons to be tested were to be stationed. This was due to the fact that the sound waves reflected from certain of the Walls would meet the oncoming waves and either increase or reduce the sound intensity so that standing sound waves were produced with the result that the sound level varied at different localities in the room. This was overcome by using a warbled sound and by the arrangement of the speakers which eliminated standing waves and gave a sound level practically uniform throughout the room.

In the operation of my invention, a generator is employed which is adapted to produce continuous audible sounds of different frequencies and intensities. These sounds are interrupted in a manner to give individual sounds of definite duration and separated by dwell periods of predetermined length. These individual sounds have been referred to in the specification and claims as sound pulses to distinguish from the resultant sound which has merely been designated as a sound and which may consist of one or more pulses.

The invention utilizes a room A in which a number of loud speakers are placed and an audiometer B which energizes said speakers. The audiometer B comprises an oscillator C which produces an alternating current of any of a number of desired frequencies. This current passes through an amplifier D and through an attenuator E from which the same is directed to a number of loud speakers 10, 11, 12 and 13 arranged in a certain manner in the room A. The alternating current produced by the oscillator is controlled by an interrupting device G which interrupts the current to produce groups of current pulses which differ in certain ways to enable the person being tested to distinguish the sounds produced in the different groups by means other than by the frequency or intensity of the sound. The interrupting device G gives to each group of current pulses a different characteristic by means of which it may be recognized and which has been referred to as a sound pattern. The current pulses are further modified by frequency modulation procured by a frequency modulating device F. A power supply unit H furnishes suitable current for operating the audiometer. The oscillator C, amplifier D, frequency modulating device F and interrupting device G form a generator of modulated alternating current pulses which will be referred to by the reference character I. These parts will now be described in detail.

The room A shown in Fig. 1 is representative of an ordinary school room having four walls 14, 15, 16 and 17 with the usual windows 18 and doors 19. The walls 14, 15, 16 and 17 as is the case with the ordinary school room are hard plastered and provide good surfaces for reflecting sound waves. While it is not necessary that all of the walls be reflecting walls, at least the wall 17 should be so constructed. In the room are arranged a desk 430 and chair 4131 for the instructor and the requircd number of combination seats and desks 402 for the students or the persons to be tested by the apparatus and method. These seats and desks are arranged in the middle of the room with aisles 403 therebetween and with aisles 404 between said seats and the walls 14 and 16. The seats 482 do not extend up to the wall so that a space 465 is formed between said seats and the Wall 15 in which the desk 4th) and chair 461 are situated.

The oscillator C consists of a thermionic tube 21 having a grid 22, a plate 23, a cathode 24 and a heater 25 for said cathode. The tube 21 is coupled with a portion of an inductance 26 and any of a number of condensers 27, 28, 29, which are connected in series to form a resonant circuit. This construction provides an oscillator which produces an alternating or pulsating current whose frequency depends upon the relation between the inductance and condenser connected in the circuit. For connecting a certain portion of the inductance 26 and any one of the various condensers in the circuit, two multiple contact switches and 66 are employed. The switch 35 has a number of fixed contacts 36, 37, 38, 39, 40, 41 and 42 and a movable switch arm 70. The switch 60 has a number of fixed contacts 44, 45, 46, 47, 43, 49 and 50 and a movable switch arm 8%. Both of the switch arms 7% and 30 are mounted on the same shaft, which is diagrammatically indicated at 151 in the drawing, and travel in unison so that certain of the sections of the inductance 26 are always connected to certain of the condensers 27 to 29 inclusive. The inductance 26 is provided with five taps 3t 31, 32, 33 and 34. A conductor is connected to the tap 3t) and also to the contact 36. Another conductor 56 is connected to the tap 31 and to both of the contacts 37 and 33. Another conductor 57 is connected to the tap 32 and to both of the contacts 39 and 40. A conductor 54 is connected to the tap 33 and to the contact 41 while still another conductor 53 is connected to the .tap 34 and contact 42. One side of each of the condensers 27 to 29 inclusive is connected to a conductor 58 which is grounded. The other side of the condenser 27 is connected by means of a conductor 61 with contacts 48,

49 and 50. The other side of condenser 28 is connected by means of a conductor 62 with contacts 46 and 47 while the other side of condenser 29 is connected by means of a conductor 63 with contacts 44 and 45. The inductance 26 is connected by means of a conductor 69 with a resistor 71 which, in turn, is connected by means of a conductor 72 with the grid 22 of tube 21. The cathode 24 of this tube is connected by means of a conductor 73 with a resistor 74 which, in turn, is connected by means of a conductor 75 with both of the switch arms 70 and 80. It will thus be seen that one of the condensers 27 to 29 inclusive and a portion of the inductance 26 are connected in series in the grid circuit of the tube 21.

For the purpose of providing current for operating the invention, tl e power supply H is employed. This power supply includes a transformer 76 having a primary 77 adapted to be connected to a suitable source of alternating current. This transformer also has a secondary 78 which is provided with a center tap 79 and which is connected to ground by means of a conductor 81. The ends of this secondary are connected by means of conductors 82 and 83 to the plates 84 and 85 of a rectifier tube 36. The filament 87 of this tube is connected by means of a conductor 88 to a secondary winding 89 on the transformer 76. This winding is connected to a condenser 91 by means of a conductor 92. Condenser 91 is grounded through a conductor 93. Another conductor connects the other side of the filament 87 to conductor 92. The output of the tube 86 is directed through a filter system 94 and from which direct current is furnished to the audiometer by means of a conductor 95. The filter system 94 includes two resistances 96 and 97 and in addition to the condenser 91, two other condensers 98 and 99. Both the condensers 9S and 99 are grounded through conductors ltll and 1.62. The resistance 96 is connected to the conductor )2 and by means of conductor 103, to the resistance 97. This resistance is, in turn, connected to the conductor 95. Condenser 98 is connected by means of a conductor 164 with the conductor 103 while condenser 99 is directly connected to the conductor 95.

The plate 23 of tube 21 is connected to a filter system 105 which consists of two condensers 166 and 107 and two resistors 169 and 111. The plate 23 is connected by means of a conductor 112 to the resistor 199 which is connected by means of a conductor 113 to the resistor 111. A conductor 114 connects the conductor 112 with condenser 106 and another conductor 115 connects the conductor 113 with condenser 107. Both condensers 166 and 107 are grounded through conductors 117 and 113. Plate voltage is procured for the tube 21 from the con ductor 95. A branch conductor 121 is connected to the conductor 95 and to the resistance 111 which in turn is connected to conductor 113.

Due to the fact that certain frequencies are more audible to the human car than other frequencies, it becomes necessary to equalize the output of the oscillator C in accordance with the frequencies of the currents produced thereby. For this purpose, a series of equalizing resistors 122, 123, 124, 125, 126, 127 and 123 are on ployed. Also, an additional multiple contact switch 131 is used. This switch has a number of contacts 132 to 138 inclusive which are connected to the various resistors 122 to 128. Switch 131 also includes a switch arm 141 which is adapted to make contact with any of said contacts. The three switches 35, 60 and 131 are all ganged and the switch arms '70, 80 and 141 of the same are all connected to the common shaft 151, previously referred to. All of the resistors 122 to 129 inclusive are connected to a common conductor 152 which will be subsequently referred to. The switch arm 141 is connected by means of a conductor 153 to the conductor 75 which is associated with switches 35 and 61?.

The amplifier D is merely shown diagrammatically and a single stage has been illustrated. It can, however, be readily comprehended that any type of amplifier may be employed and as many stages as found necessary utilized. The amplifier D includes a thermionic tube 154 having a grid 155, a plate 156, a cathode 157 and a heater 158 for the same. The conductor 152 previously referred to, is connected directly to the grid 155 of the tube 154. A resistor 164 is connected by means of a conductor 165 to the conductor 152. Said resistor is further connected by means of a conductor 166 to ground. The cathode 157 of this tube is connected by means of a conductor 159 to a condenser 162 and a resistor 163. This condenser and resistor are in turn connected by means of conductors 167 and 168 to the conductor 166 and thus grounded. The plate 156 of the tube 154 is connected by means of a conductor 169 with one side of the primary 171 of an output transformer 172. The other side of this primary is connected by means of a conductor 173 with the conductors 121 and 95. Thus, plate voltage is furnished through the primary of this transformer to the plate 156 of the tube 154. The secondary 174 of the transformer 172 is connected to conductors 175 and 176 which impresses the amplified voltage upon the attenuator E.

The attenuator E comprises six resistors 177, 178, 179, 188, 181 and 182 which are connected in series by means of conductors 183, 184, 185, 186 and 187. The resistor 177 is further connected to the conductor 176. In addition to the siX resistors 177, 178, 179, 180, 181 and 182, six other resistors 142, 143, 144, 145, 146 and 147 are employed. These resistors are all connected at one end to the conductor 175 from the secondary 174 of transformer 172. The other ends of these resistors are connected by means of conductors 148 to the conductors 183, 184, 185, 186 and 187 previously referred to. The attenuator E also includes a selector switch 195 which comprises a number of contacts 196, 197, 198, 199, 208 and 281. These contacts are connected by means of conductors 283 to the various conductors 183, 184, 185, 186 and 187 respectively. Said switch includes a switch arm 209 which is adapted to make contact with any of the said contacts. The loud speakers 10, 11, 12 and 13 are connected in parallel and to a conductor 211 connected to switch arm 209 and further to the conductor 175 which is connected to the secondary 174 of transformer 172.

The frequency modulating device F consists of a number of condensers 218. These condensers are all connected together in series by means of conductors 219. In addition to these condensers a number of condensers 22!) are employed. These condensers are connected together in series by means of conductors 221. In addition, other condensers 222 are employed which are connected together in series by means of conductors 223. All of these condensers are controlled by a multiple switch 224 which has six decks 225. These decks are all ganged and each has a movable arm 226 and three contacts 227, 228 and 229. The condensers 218 are connected by means of conductors 230 to the contacts 227. In a similar manner the condensers 220 are connected by means of conductors 231 to the contacts 228 and likewise condensers 222 are connected by means of conductors 232 to the contacts 229. The end condensers 218, 220 and 222 are grounded by means of conductors 279.

For operating the frequency modulating device F a rotary switch 233 is employed which has a rotating arm 234 and a number of fixed contacts 235 and 236 arranged in opposition to one another. Each of the contacts 235 is connected to the corresponding contact 236 by means of conductors 237. The conductors 237 are connected by means of other conductors 238 to the switch arms 226 of the switch 224. Intermediate the contacts 235 and 236 are contacts 242 and 243. The contact 242 is connected to a conductor 244 which is connected to the switch arm 234 and to the conductor 75 and to the conductor 153 between the arms 80 and 141 of switches 35 and 131. The

contact 243 is connected to one of the conductors 238. The arm 234 of switch 233 is mounted on a shaft diagrammatically indicated at 239 which is driven by a transmission 240 operated by an electric motor 241. The transmission 240 is so designed that the shaft 239 operates approximately 360 R. P. M. The condensers 218 vary in capacity so that when the shaft 239 is uniformly rotated the output of the oscillator C is frequency modulated.

The interrupting device G utilizes a shaft diagrammatically indicated at 246 which is connected to and driven by the transmission 240. This shaft operates at substantially 10 R. P. M. The said shaft carries a drum 247 having on the same a cam 248. Operating in conjunction with the said drum and cam are switches 249, 250, 251 and 252. Each of these switches has a fixed switch arm 253 and a movable switch arm 254. On the arm 254 is a cam follower 255. These various switches are normally closed and when the cam 248 engages the cam followers 255 the said switches are opened for a period of time depending upon the length of the cam 248 and the speed of the shaft 246. The switches 249, 250, 251 and 252 are all connected in series by means of conductors 256 which go from the fixed switch arm of one switch to the movable switch arm of the following switch. The movable switch arm 251 of the switch 249 is connected by means of a conductor 257 to the conductor 113 previously referred to and which is connected to the plate of the oscillator tube 21 through resistors 109 and 111. The fixed contact 253 of switch 252 is connected by means of conductor 253 to ground. It will readily be comprehended that the switches 249, 251), 251 and 252 beingnormally closed from a by-pass circuit conducting the output of the oscillator to ground and thus rendering the oscillator inoperative as to furnishing operating current for the loud speakers 10, 11, 12 and 13. When, however, the cam 248 reaches the cam follower 255 of switch arm 254 the switch of which said arm forms a part is opened and the output from oscillator C utilized for procuring energy for energizing the said loud speakers. The drum 247 is stopped each time the same completes a single revolution. This is accomplished by means of a switch 259 constructed similarly to the switches 249, 250, etc. One side of this switch is connected by means of a conductor 260 to one side of a line 261 connected to a source of alternating current. The other side of this line is connected by means of a conductor 262 to one terminal of the motor 241. The other terminal of this motor is connected by means of a conductor 263 to the other arm of switch 259. A push button 264 is connected across the conductors 263 and 260. This push button serves to momentarily actuate the motor 241 and to start the drum 247 to rotate. As soon as the cam 243 passes the cam follower of switch 259 the push button 264 is released and the drum then continues to rotate until cam 248 subsequently opens switch 259. The drum 247 normally causes the successive openings of the switches 249, 250, 251 and 252 which would produce in the loud speakers 10, 11, 12 and 13 four tone pulses of a predetermined duration with certain intervals of quiet therebetween. The sound pattern of the tone so produced can, however, be varied to procure three, two or one such sound pulses instead of four. This is accomplished by means of a selector switch 265 which consists of a movable arm 266 and four contacts .267. The contacts 267 are connected by means of conductors 268 to the various conductors 256. The arm 266 is connected by means of a conductor 269 to the conductor 257. The switch arm 266 has an elongated contact 270 formed on the same and which operates to engage the contacts 267. As this latter contact moves with the switch, one, two or three of the contacts 267 are closed and the interrupting device G caused to produce four, three, two or one tone pulses in the speakers.

The operation of the invention is as follows: The tube 21 when energized functions as an oscillator with a grounded plate. Output from the oscillator is fed from the cathode 24 of the tube through the conductors 75 and 152 and to the grid 155 of the amplifier tube 154. The frequencies of the oscillating current is principally determined by the amount of inductance of the coil 26 included in the circuit by switch 35 and by the amount of capacity introduced into the circuit by the switch 60. The amplitude of the alternating current so produced is equalized for the various frequencies by the resistors 122, etc., controlled by switch 131. In addition to the capacity procured from condensers 27, 28 and 29, additional capacity is introduced into the circuit from the condensers 218, 220 and 222. This is accomplished by the rotary switch 233. This switch is driven at a uniform rate of speed and successively and gradually increases and decreases the total capacity so introduced. The capacities 218, 220 and 222 are selected as to value to cause the frequencies to rise and fall according to a predetermined rate. This has the effect of frequency modulating the alternating current generated by the oscillator C. The interrupting device G includes the switches 249, 259, 251 and 252 which are connected in series in a circuit between the plate 23 of the oscillator tube 21 and ground. When all of these switches are closed the plate is grounded rendering the tube 21 inoperative. As the cam 248 successively engages the cam followers 255 the said switches are successively opened causing alternate periods of operation and cessation of operation of the oscillator which produces sound pulses in the loud speakers 10, 11, 12 and 13. There being four switches controlling the energization of oscillator C, four pulses may be produced in succession during a single revolution of the shaft 246. Paralleling the switches 249 to 252 is the switch 265. This switch permits of maintaining the plate grounding circuit closed even though the switches 249 to 252 are opened. Thus three, two or one pulse may be procured by properly manipulating switch 265. Switch 259 deenergizes motor 241 after shaft 246 has made a single revolution, which may only be again energized by actuating push button 264 long enough to cause cam 248 to move off from the cam follower controlling switch 259. The frequency modulated alternating current is amplified by the amplifier D and passes through the attenuator E, and energizes the loud speakers to 13. When it was first attempted to use loud speakers for the simultaneous testing of a number of persons it was found that the sound waves were reflected by the walls and met the oncoming sound waves from the speaker. This caused the sounds to be increased in intensity at some locations and decreased in intensity at other locations within the room. In other words standing waves were produced. This effect could not be overcome with any arrangement of the speakers. Comparable tests could hence not be made. This was overcome by frequency modulating the alternating current so a sound of substantially the desired frequency was obtained which had a slight amount of warble to it. This warble, however, was slight enough so that the person being tested would never discern the difference and to all intents and purposes was aware only of hearing a sound of the desired pitch. The reflected sound waves in such case never meet the oncoming sound waves at the same locality and so no standing waves are produced. By directing the sounds toward a reflecting wall the reflected waves are utilized to augment the direct waves in a manner such that almost uniform sound intensity can be procured throughout the entire area in which the seats of a school room are situated. The best results were attained when the loud speaker closest to the reflecting wall was situated a distance therefrom less than the velocity of sound divided by the frequency of modulation of the alternating current.

Termination of the sound pulses in the form of the invention shown in Fig. 1 is procured by disabling the oscillator tube 21. The same results can be procured by controlling the grid bias on the amplifier tube. A circuit utilizing this system is shown in Fig. 3. Due to the similarity of construction the description of the same will not be repeated and the same reference numerals followed by the suffix a will be used to designate the corresponding parts. In this form of the invention the grid resistor 164a is connected by means of a conductor 500 to a filter 501 comprising a resistor 502 and a condenser 503. The condenser 503 is grounded by means of a conductor 504. Grid bias is furnished from the power supply Ha. For this purpose a tap 505 is taken on the secondary 78a of transformer 76a. A conductor 506 is connected to the tap 505 and to a current limiting resistor 507. This resistor is in turn connected to a conductor 508 which is connected to resistor 502. Between the tap 505 and ground a resistor 509 is connected which determines the amount of the bias for the grid a of tube 154a. This resistor is shunted by a bypass condenser 510. The bias is of such a value that the tube 154a is biased to cut off and normally fails to function. For rendering the tube operable the switches 249a, 259a, 251a and 252a are employed. In this form of the invention said switches are normally open and are adapted to be closed when the cam 248a engages the proper cam follower 255a. These switches are all connected in parallel by means of the conductors 256a, which are connected to the switch arms 254a. The conductors 268a connect the other switch arms 252a to the contacts 267a of the switch 265a. The conductors 257a and 269a are connected to the conductor 508.

The method of operation of the form of invention shown in Fig. 3 is as follows: Normally the tube 154a is disabled due to the high grid bias on the grid 155a of the same. When any of the contacts 267a are disconnected from the switch arm 266a the circuit through the corresponding switch 249a, etc., is open. Upon closing of said switch by the cam 248a a path is formed for the grid current which includes conductor 508, conductor 25701, the conductors 256a and conductor 258a to ground. The opening of the circuits by the switch 265a immediately disables the corresponding switch of switches 249a, etc., thus controlling the number of pulses produced during each rotation of the shaft 247a. The results procured with this form of the invention are exactly the same as with the other form of the invention excepting that the control is procured through the amplifier tube instead of the oscillator tube.

The advantages of the invention are manifest. The conglomeration of wires and circuits used with other forms of test systems for testing the hearing acuity of persons is eliminated with the instant invention. With the invention the sound intensity is substantially constant throughout the entire room in which the test is being conducted so that numerous persons may be conveniently tested. By the use of ear closures the ears of the patients can be selectively tested.

Changes in the specific form of the invention, as herein described, may be made within the scope of what is claimed without departing from the spirit of the invention.

Having described the invention, what is claimed as new and desired to be protected by Letters Patent is:

1. In a group hearing test system for simultaneously and collectively testing a number of persons, an oscillator for generating an alternating current suitable for producing an audible test tone, an interrupting device connected to said oscillator and varying the continuity of the alternating current to produce current pulses of certain duration, a periodically operated frequency modulating device connected to the oscillator and periodically varying the frequency of the alternating current, means for causing the periods of the modulations produced by said modulating device to be shorter than the lengths of said pulses, and a loud speaker energized by said current pulses and converting said current pulses into warbled tone pulses uniformly audible to the persons tested.

2. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, the combination of a loud speaker in said room adjacent said second named wall and emanating sound waves directed toward said reflecting wall and a generator of frequency modulated alternating current pulses for energizing said loud speaker to produce warbled tone pulses.

3. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, the combination of a plurality of loud speakers in said room adjacent said second named wall and emanating sound waves directed toward said reflecting wall and a generator of frequency modulated alternating current pulses for energizing said loud speakers to produce warbled tone pulses.

4. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, the combination of a plurality of loud speakers in said room all emanating sound waves directed toward said reflecting Wall, some of said speakers being situated adjacent said second named wall and a generator of frequency modulated alternating current pulses for energizing said loud speakers to produce warbled tone pulses.

5. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, the combination of a loud speaker in said room adjacent said second named wall and emanating sound waves directed toward said reflecting wall and a generator of frequency modulated alternating current pulses for energizing said loud speaker, the distance of said loud speaker from said reflecting wall being less than one-half the ratio of the velocity of sound in air divided by the frequency of modulation of said alternating current.

6. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, the combination of a plurality of loud speakers in said room all emanating sound waves directed toward said reflecting Wall, some of said speakers being situated adjacent said second named wall and others being disposed intermediate said walls, the distance of said last named loud speakers from said reflecting wall being less than one-half the velocity of sound in air divided by the frequency of modulation of said alternating current.

7. In a group hearing test system for use in a room having acoustical reflecting walls for collectively testing a number of persons distributed about said room, the combination of a sound producing device disposed at one locality in said room and emanating sound waves toward certain of said walls, and an electric generator of groups of alternating current pulses comprising components of different frequencies all within a predetermined range, said pulses energizing said sound producing device to produce groups of tone pulses of varying frequency according to predetermined sound patterns.

8. In a group hearing test system for use in a room having an acoustical reflecting Wall and a wall opposite thereto, a number of seats in said room arranged diflerent distances from said walls for positioning the patients to be tested in a manner facing the second named wall for testing a number of persons distributed about said room, the combination of a number of loud speakers in said room all emanating sound waves directed toward said reflecting wall and a generator of frequency modulated alternating current pulses connected to and energizing said loud speakers.

9. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, a number of seats in said room arranged diflerent distances from said walls for positioning the patients to be tested in a manner facing the second named wall for testing a number of persons distributed about said room, the combination of a number of loud speakers in said room all emanating sound waves directed toward said reflecting wall, an audiometer in said room connected to and energizing said speakers and a frequency modulating device connected to said audiometer and varying the frequency of the alternating current generated thereby to cause said loud speakers to provide a warbled tone.

10. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto, a number of seats in said room arranged different distances from said walls for positioning the patients to be tested in a manner facing the second named wall for testing a number of persons distributed about said room, the combination of a number of loud speakers in said room all emanating sound Waves directed toward said reflecting wall, an audiometer connected to and energizing said speakers and a frequency modulating device connected to said audiometer and varying the frequency of the alternating current generated thereby to a degree to cause said loud speakers to produce a warbled tone varying in pitch above and below the pitch for which the audiometer is set an amount substantially aurally imperceptible to the patient.

11. In a group hearing test system for use in a room having an acoustical reflecting Wall and a wall opposite thereto for testing a number of persons distributed about said room, and combination of a plurality of loud speakers in said room adjacent said second named wall and emanating sound waves directed toward said reflecting wall, an audiometer connected to and energizing said speakers and a frequency modulating device connected to said audiometer and varying the frequency of the alternating current generated thereby to cause said loud speakers to produce a warbled tone.

12. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto for testing a number of persons distributed about said room, and combination of a plurality of loud speakers in said room adjacent said second named wall and emanating sound Waves directed toward said reflecting Wall, an audiometer within said room connected to and energizing said speakers and a frequency modulating device within said room connected to said audiometer and varying the frequency of the alternating current generated thereby to a degree to cause said loud speakers to produce a warbled tone varying in pitch above and below the pitch for which the audiometer is set an amount substantially aurally imperceptible to the patient.

13. In a group hearing test system for use in a room having an acoustical reflecting wall and a wall opposite thereto for testing a number of persons distributed about said room, and combination of a plurality of loud speakers in said room adjacent said second named wall and emanatsound waves directed toward said reflecting wall, an audiometer within said room connected to and energizing said speakers, said audiometer including an oscillator having an inductance and a condenser connected to resonate at a predetermined frequency and producing an alternating current of such frequency, a bank of condensers of graduated values and time controlled means for successively and periodically connecting said condensers to said first named condenser to frequency modulate said alternating current and cause said loud speakers to produce a warbled tone.

14. An audiometer including an oscillator having an inductance and a condenser connected to resonate at a predetermined frequency and producing an alternating current of such frequency, a loud speaker operated thereby, variable condenser means connected to said condenser and time controlled means connected to and periodically varying the capacity of said variable condenser means connected to said first named condenser to frequency modulate said alternating current and cause said loud speaker to produce a warbled tone.

15. An audiometer including an oscillator having an inductance and a condenser connected to resonate at a predetermined frequency and producing an altrenating current of such frequency, a loud speaker energized there- H by, a bank of condensers of graduated values'and time controlled means for successively and periodically connecting said condensers to said first named condenser to frequency modulate said alternating current and cause said loud speaker to produce a warbled tone.

Marks Aug. 24, 1926 Karnes July 26, 1938 12 Kaye Aug. 15, 1939 Cubert Jan. 21, 1941 Davis Aug. 11, 1942 Garity et a1. Oct. 13, 1942 Roetken June 29, 1943 Strommen Feb. 12, 1946 Winlund Jan. 7, 1947 Allison May 24, 1949 Reger Jan. 9, 1951 

